In the past, bodies of ferromagnetic material have been inspected by a method such as the flux leakage method as taught, for example, in U.S. Pat. No. 3,091,733 (May 28, 1963, Fearer et al), 4,468,619 Aug. 28, 1984, Reeves), and 4,602,212 (Jul. 22, 1986, Hiroshima et al). In this method, the metal is magnetized in a direction parallel to its surface. At defects or where regions of the metal body are not uniform, some magnetic flux passes into the air and may be detected by sensors located nearby, thus giving an indication of the presence of faults, non-uniformity, etc.
U.S. Pat. No. 4,107,605 (Aug. 15, 1978, Hudgell) discloses an eddy current technique for detecing abnormalities in a pipeline of a ferromagnetic material. The eddy current probe includes a plurality of spiral sensing coils placed with their axes normal to the surface of the pipeline wall and connected on four legs of an AC bridge, thus compensating for lift-off. A biasing magnetic field by a permanent magnet permits distinguishing internal from external defects in weakly ferromagnetic tubes by comparing outputs from systems with and without biasing field. A partial magnetic saturation is achieved but the sensing coils are placed at one location with a saturation level. No multiple saturation levels are employed.
U.S. Pat. Nos. 3,952,315 (Apr. 20, 1976, Cecco) and 2,964,699 (Dec. 6, 1960, Perriam) describe eddy current probes for use of testing weakly ferromagnetic tubes. They both include magnetic saturation means. Their eddy current sensing coil assembly is located at a place with one magnetic saturation level.
In U.S. Pat. Nos. 2,992,390 (Jul. 11, 1961, de Witte) and 3,940,689 (Feb. 24, 1976, Johnson, Jr.) special electromagnetic ways of generating magnetic fields are taught in connection with the eddy current testing in that de Witte uses uniquely designed cores for transmit-receive coils and Johnson, Jr. employs a solenoid wound about a core of a substantial length. There are no teachings about two or more levels of magnetization.